The present invention relates generally to dimming electronic ballasts used to power gas discharge lamps. Specifically, this invention pertains to a 0-10 volt dimming control interface for an electronic ballast that includes components that will prevent the dimming control interface from being damaged if it is accidentally connected to an external AC power source. More specifically, the dimming control interface of the present invention includes circuits that limit voltages and currents present in the dimming control interface circuit to safe levels when the external AC power source is connected to the dimming interface circuit.
Generally, a 0-10 volt dimming control interface is designed to be connected to a DC control voltage, while the ballast itself is designed to be connected to an external AC power source. A conventional 0-10 volt control interface is designed to deliver a small current preferably ranging from about 300 .mu.A to 500 .mu.A. Consequently, a typical DC control voltage may be obtained from a variable resistor, a shunt controller, or a more sophisticated light-level controller. Shunt controllers control ballasts by setting the voltage between the control input terminals to a level ranging from 0 volts to 10 volts. If the shunt controller is disconnected from the dimming control interface, the open-circuit voltage between the control input terminals will rise to approximately 10 volts.
To allow these connections, dimming ballasts usually include a pair of dimming control input terminals designed to be connected to the DC control voltage, and a pair of AC power input terminals designed to be connected to the external AC power source. Sometimes, however, the AC power source is accidentally connected to the dimming control input terminals rather than the AC power input terminals. As a result, the dimming control interface and the ballast may be damaged by excessive currents and voltages generated by the AC power source. Replacement of the damaged dimming control interface and/or the ballast is an expensive proposition. Thus, there is a need for a dimming control interface that may be connected to a DC control voltage, as well as, an external AC power source without being damaged.
To complicate matters even more, a dimming control interface may be connected to several different ballasts through a single pair of dimming control input terminals. When the external AC power source is connected to such a dimming control interface, excessive currents and voltages generated by the AC power source may damage several ballasts. Replacement of multiple dimming control interfaces and/or ballasts is accordingly more expensive and undesirable. Thus, there is an additional need for a dimming control interface that may be connected to an external AC power source without being damaged.
Although dimming control interface circuits are well known in the art, there are no known dimming control interfaces that are capable of withstanding a sustained connection to an external AC power source without being damaged. For example, one known dimming control interface is described in U.S. Pat. No. 5,751,188 issued to Mortimer on May 12, 1998 and entitled "Universal Input Dimmer Interface." This patent teaches a dimming control interface that may be connected to an external DC control voltage or an external pulse-width modulated control voltage. In either case, the dimming control interface includes circuitry for generating an electrically isolated control voltage at output terminals on the dimming control interface in response to the input control voltage applied at input terminals on the dimming control interface, whether the input control voltage is the DC control voltage or the pulse-width modulated control voltage. This patent fails to teach or suggest the use of any circuits for the purpose of limiting currents and voltages present in the dimming ballast when an external AC voltage is applied to the dimming control input terminals.
Another example is described in U.S. Pat. No. 5,583,402 issued to Moisin et al. on Dec. 10, 1996 and entitled "Symmetry Control Circuit and Method." This patent also teaches a dimming interface circuit designed to be connected to either a DC control voltage or a pulse-width modulated voltage signal. As was the case with the '188 patent, this patent fails to teach or suggest the use of circuits designed to limit currents and voltages present in the dimming ballast when an AC power source is connected.
Two additional examples of dimming control interfaces are described in U.S. Pat. No. 5,089,751 issued to Wong et al. on Feb. 18, 1992 and entitled "Fluorescent Lamp Controllers with Dimming Control," and U.S. Pat. No. 5,003,230 issued to Wong et al. on Mar. 26, 1991 and entitled "Fluorescent Lamp Controllers with Dimming Control." Neither of these patents teach a dimming control interface that includes protection from connection of the dimming input terminals to an external AC power source.
In addition, there are two known commercial dimming control interfaces. A portion of a one such prior-art dimming control interface circuit is shown in FIG. 1. A small-signal PNP transistor Q1 is connected to a biasing circuit (not shown) so that the transistor will supply about 400 .mu.A current to a shunt controller (not shown) connected between control input terminals J1 and J2. A zener diode D1 prevents the voltage between terminals J1 and J2 from rising above about 10 V if no shunt controller is connected to the circuit. An isolated coupling circuit (not shown) may be connected between terminals J3 and J5. The isolated coupling circuit sends a dimming control signal to a ballast inverter circuit (not shown). A resistor R1 provides protection for the interface circuit if the control terminals should accidentally be connected to a low voltage source such as a 24 VAC control line. R1 typically has a value of about 1 k.OMEGA., so the 0.4 V voltage drop due to the 400 .mu.A current is small in comparison with 10 V, and can be compensated for. It is impractical to make R1 large enough in value to withstand ac power line voltages because the voltage that would then be dropped across it due to the 400 .mu.A current would be large in comparison with 10 V.
A portion of another commercial dimming control interface is shown in FIG. 2. A floating 12 V dc power supply (not shown) is connected between terminals J14 and J15, with the positive terminal connected to J14. Resistors R2 and R3 have a value of 10 k.OMEGA.. They supply current to control input terminal J11 through a high voltage diode D2. A high-voltage N-Channel power MOSFET transistor Q2 is biased on by resistors R2 and R3 when terminal J11 is positive with respect to J12, which is the case during normal operation. During normal operation, the voltage between terminals J13 and J15 is about 0.6 V less than the voltage between terminals J11 and J12. The voltage between terminals J13 and J15 is coupled to a regulator circuit consisting of an operational amplifier U1, resistors, R4, R5 and R6, a capacitor, C1, and a current sensor circuit 10. The input terminals of an optocoupler (not shown) are connected between terminals J16 and J15. The current sensor provides a signal that is proportional to the lamp current. This signal provides negative feedback even though it is connected to the positive terminal of U1 because the optocoupler is connected to the ballast inverter (not shown) in a way that decreases lamp current when the input current to the optocoupler increases.
If the voltage between terminals J11 and J12 becomes negative, Q2 is turned off, and this protects the regulator circuit. If the voltage between terminals J11 and J12 becomes greater than about 11 V, diode D2 becomes reverse biased, and this also protects the regulator circuit. If the blocking capabilities of D2 and Q2 were each about 600 V, then the regulator circuit would be protected if terminals J11 and J12 were connected across a 277 V power line that had no transients. This dimming interface circuit is more robust than conventional 0-10 V dimming interfaces, but it is excessively costly because it includes a high-voltage diode and a high-voltage power MOSFET transistor. In addition, there are transient voltages that commonly occur on ac power lines, and diode D2 and transistor Q2 could be destroyed by these transients. Furthermore, the active dimming range is from 0.1 V to 6.6 V, while many shunt controllers, particularly manual types with slide controls, are designed to operate from about 1 to 9 V. Finally the current sourced by the control terminals is only 48 .mu.A at 10 V, and some controls require 200 .mu.A.
What is needed, then, is a dimming control interface that provides a current ranging from 300 .mu.A to 500 .mu.A in response to a DC control voltage ranging from 0-10 volts, and limits the voltages and currents present in the dimming control interface to safe levels when an external AC power source is accidentally connected to the dimming control input terminals.